1. Field of the Invention
The present invention relates to an image processing apparatus, and particularly to an image processing apparatus which is suitable for performing inter image calculations between two processed images of a single subject.
2. Description of the Related Art
Inspection of a single subject, by viewing two or more images thereof and investigating the differences among the images, is being performed in a variety of fields.
For example, in the field of industrial manufacture, an image of a product immediately after manufacture and an image of the same product after durability tests have been administered thereon are comparatively viewed. By paying attention to parts where a large difference exists, parts of the product at which durability should be improved are investigated. In the field of medicine, a physician may comparatively view radiation images of a patient which have been taken at different times. Thereby, occurrence of new disease, a progression of disease, or a treatment state can be understood. Appropriate treatment protocols are investigated based on these understandings.
In the case that two or more images, which are the objects of comparative viewing, are output, it is common for the images to be simply arranged side by side. However, the differences among the images are the areas of greatest interest to a viewer. In the case that two images are simply arranged side by side as described above, it becomes difficult to find the differences as they get smaller. Therefore, there is demand for improved performance in comparative viewing of images.
Therefore, extraction and emphasis of the differences between two images are being performed. This is accomplished by inter image calculation, including subtraction processes, in which positions of structural components (anatomically characteristic positions) in two images are made to correspond to each other (refer to, for example, U.S. Laid-open No. 20010002934). Particularly in the field of medicine, a temporal series subtraction technology has been proposed (refer to A. Kano, K. Doi, H. MacMahon, D. Hassell, M. L. Geiger “Digital Image Subtraction of Temporally Sequential Chest Images for Detection of Interval Change”, Med. Phys. 21(3), March 1994, 453-461[1]). This technology generates a difference image between radiation images taken in a temporal series. Aid in diagnosis by observing the generated difference image along with the radiation images taken in a temporal series is being attempted.
A viewer of images is enabled to surely recognize the differences therebetween, by only the differences between images being extracted and emphasized in this manner. Therefore, for example, in the field of medicine, it can be thought that progression or a treatment state of a diseased portion being overlooking will be prevented.
It is desirable that the positions of structural components within images are positionally aligned so that they correspond between images which are to become objects of comparative viewing. This is because the positions of the structural components differ between images, due to changes in the posture of a subject, a difference in an imaging apparatus, and the like. As an example of this positional alignment, a two step process is known (refer to United States Patent Publication No. 20030016853. The two step process involves a first positional alignment operation and a second positional alignment operation. The first positional alignment operation is a global transform (for example, Affine Transform) accomplished by parallel movement, rotation, magnification and reduction of the two images. The second positional alignment operation first divides the images into a plurality of local regions following the first positional alignment operation. Then, the images which have been matched in the corresponding local regions are subjected to nonlinear distortion transform (warping) by curve fitting (for example, a two dimensional tenth degree polynomial).
However, even if the structural components of two images are completely positionally aligned by the process disclosed in Japanese Unexamined Patent Publication No. 8(1996)-335721, there are cases in which specific signal values representing the density or the luminance of each correspondent structural component in the two images do not match. In these cases, artifacts due to the differences in signal values are generated in an inter image calculation image (difference image) obtained by inter image calculation.
Images which are the objects of comparative viewing, that is, which are subject to inter image calculation, are not originally obtained with inter image calculation as an objective. Image processes according to image processing conditions appropriate for each image are administered thereon, so that each image per se is reproducible as a visible image suitable for viewing. Particularly in the case of images obtained in a temporal series, they are obtained with the objective of grasping the state of a subject at the time of image obtainment. Therefore, image processes are administered with image processing conditions which are set for the particular obtained image. Accordingly, it is often the case that images which are the objects of inter image calculation have undergone image processes with image processing conditions which are different from each other. In these cases, the possibility of artifact occurrence is extremely high, because the signal values representing the density or the luminance of each correspondent structural component in the two images do not match.
When images are saved, fundamentally, an image to which image processes have been administered (processed image) and the image prior to image processes having been administered thereon (unprocessed image) are saved. The aforementioned problem can be prevented by performing the inter image calculation employing the unprocessed images.
Recently, however, various standards have been adopted for the transmission and reception of images. Particularly in the field of medicine, standards such as DICOM have become prevalent, and transmission and reception of images via networks and the like have become facilitated. Accompanying this development, images are being exchanged in a variety of formats and states. Images for which only processed images exist are also being exchanged, necessitating inter image calculation between a processed image and an unprocessed image, or between two processed images.